The present invention relates to an electrochemical fuel cell assembly. More particularly, the present invention relates to an electrical contacting device for a solid polymer fuel cell stack.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers.
In typical fuel cells, the MEA is disposed between two electrically conductive separator or fluid flow field plates. Fluid flow field plates have at least one flow passage formed therein to direct the fuel and oxidant to the respective electrode layers, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide support for the electrodes.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given plate may serve as an anode plate for one cell and the other side of the plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is typically held together in its assembled state by tie rods and end plates. A compression mechanism is generally required to ensure sealing around internal stack manifolds and flow fields, and also to ensure adequate electrical contact between the surfaces of the plates and membrane electrode assemblies to provide the serial electrical connection among the fuel cells which make up the stack.
In most fuel cell assemblies, current is drawn from the fuel cell stack via a pair of bus plates, one of which is positioned at each end of the fuel cell stack. The fuel cells are stacked between the bus plates, which are typically formed of copper or coated copper. Very often, individual cells of the stack are contacted for monitoring individual cell voltages or currents, and/or for control or charging/discharging purposes. In most cases, these electrical contacts are not intended to carry the entire stack current, but are to be capable of providing electrical connection to individual fuel cells or groups of cells.
In mass production, an electrical contacting device is needed which is easy to handle and to install, and which provides reliable electrical contact with certain components of a fuel cell stack. It may be desirable to provide, in a single device, groups of contacts that always communicate with the same type of fuel cell component within the stack, or that contact the fuel cell stack at regularly spaced intervals along the length of the stack. In general, it is preferred that most components of the fuel cell stack be electrically isolated from the surrounding environment, and not readily electrically accessible from the outside because of potential electrical shock hazards. For this reason, fuel cell stacks often have some form of electrically insulating cover or housing.
In operation, fuel cells expand and contract due to thermal variations, internal pressure changes and gradual compression of cell components over time. Thus, in a fuel cell stack where a plurality of cells is stacked and electrically connected in series, the overall stack length may vary significantly over time. Preferably, a contacting device can accommodate such dimensional changes. For example, the contacting device may be designed to have a similar thermal expansion coefficient to components of the particular fuel cell stack.
Furthermore, depending on the application, a fuel cell stack may be subject to vibration. In these situations, the contacting device will be effective if reliable electrical contact is maintained when the fuel cell is subject to vibration.
An operational fuel cell stack may generate high electrical currents. The changing electrical current may induce electromagnetic fields around the stack. Signals transmitted by the contacting device are often low both in voltage and current. Thus, unless appropriate precautions are taken, the contacting device may be subject to electromagnetic interference caused by the fuel cell stack itself and also other system components which may be present, such as electric motors and power conditioning devices.
Space is often at a premium in fuel cell systems, especially, for example, in automotive and portable applications. A contacting device for a fuel cell stack with a thin profile (that is, a sheet or board) may be easier to accommodate than a more bulky contacting device having significant thickness in all three dimensions. The contacting device may also have to withstand the operating fuel cell environment which may be, for example, a moist and/or an acidic or alkaline environment.
Thus, there is a need for a fuel cell contacting device that satisfies some, or preferably all, of the above requirements.
An electrical contacting device for a fuel cell assembly comprises a circuit board having electrically conductive regions for electrically contacting fuel cell components of the fuel cell assembly. The circuit board is preferably a printed circuit board that is flexible, rather then being substantially rigid. Typically, the circuit board comprises at least two layers, with one of the layers being an electrically insulating carrier layer upon which one or more other layers, such as electronic circuits, are disposed. Openings formed within the carrier layer may enhance the flexibility of the circuit board. The electrically conductive regions and associated electrically conductive paths are usually formed in a layer disposed upon at least one planar surface of the carrier layer. In some embodiments, electrically conductive paths are each connected to a different electrically conductive region, and the electrically conductive paths are formed in a plurality of the circuit board layers, which are preferably two layers that are associated with opposite planar surfaces of the carrier layer. An electromagnetic shielding layer may be incorporated into the multilayer circuit board.
The surfaces of the electrically conductive regions of the circuit board may be rough or uneven (that is, having projections extending from and recesses extending into the surface thereof), or may include anchors, such as, for example, pins, studs, pegs, or other attachment means, to improve or facilitate their electrical contact with the fuel cell components. The electrically conductive regions may be covered or coated with an electrically conductive film with lower material resistance than the material resistance of the underlying material. The film is preferably corrosion resistant.
An improved fuel cell assembly comprises a fuel cell stack with an electrical contacting device mounted on a face of the stack. The tow electrical contacting device preferably comprises a flexible printed circuit board with electrically conductive regions that are in electrical contact with fuel cell components of individual cells or groups of cells of the stack.
The circuit board may be fastened to the stack using any type of fastener, for example, screws, bolts, pins, pegs, clips, clamps or rivets. In addition, or alternatively, the fuel cell assembly may comprise a compression device for urging the circuit board against the face of the stack. In other embodiments, the circuit board may be in interlocking engagement with components of the stack, or may be bonded to the stack. In yet another embodiment, the circuit board may be utilized as a diagnostic tool that is attached to the stack only when it is being repaired or during regular service checks.
Preferably, the thermal expansion factors of the circuit board and the fuel cell stack are similar, and most preferably substantially identical.